The present invention relates to an abnormality detector of an injection apparatus for detecting an abnormal state of the injection apparatus, especially an abnormal state of a screw.
In an injection-molding machine, a substantial reaction force from molten resin compressed by means of a screw acts on the tip end of the screw during an injection and pressure hold operation, and the molten resin reversely flows along a groove of the screw. A check valve is generally used to prevent this. For example, the check valve includes a screw body movably disposed in a cylinder, a screw head mounted integrally on the tip end of the screw body, a ring-shaped spacer fixed to the tip end face of the screw head, and a ring fitted on the screw head and movable toward and away from the spacer along the screw head in a manner such that the outer peripheral surface of the ring is in contact with the inner peripheral surface of the cylinder.
During a metering process, the ring is separated from the spacer, and molten resin metered by means of the screw body, in rotation for a retreat, is delivered forward through a gap which is defined by the inner peripheral surface of the cylinder and a notch portion formed on the outer peripheral surface of the screw head, and opens at both ends. During injection and pressure hold, on the other hand, the ring pushed back by the reaction force of the compressed molten resin is pressed against the spacer to close the spacer-side open end of the gap, thereby preventing the molten resin from reversely flowing.
If a molding operation is performed for a long period of time, with use of a resin mixed with glass fibers, and the like, or other high-rigidity resins, wear is produced between the sliding contact surfaces of the ring of the check valve and the cylinder. If the screw is rotated or moved before the cylinder is fully heated, moreover, the check valve may possibly be rendered immovable in the cylinder or broken by the solidified resin remaining in the cylinder.
In such a case, the molten resin reversely flows during the injection and pressure hold, so that normal molding operation cannot be effected, and resulting products are subject to "shrink marks" or variation in size. If a high-pressure injecting operation is started without noticing an extreme damage to the check valve, moreover, the molten resin may possibly reversely flow in bulk, and escape from a hopper, or the like, into which pellets are introduced, thereby entailing a very dangerous situation.
In order to prevent such an accident, therefore, it is desired that the check valve should be inspected periodically. To attain this, however, the screw must be removed, thus requiring a very troublesome amount of work. If defective molding is caused by wear of the check valve, furthermore, it is very difficult to detect the cause.